Multiple Input Multiple Output (MIMO) is a core technology for 4th Generation (4G) communication and 5th Generation (5G) communication. In a Long Term Evolution (LTE)/LTE-Advanced (LTE-A) system, codebook-feedback-based closed loop MIMO becomes the mainstream and is fully developed. However, in a 5G massive MIMO system, an uplink/downlink channel reciprocity-based non-codebook closed loop MIMO technology will be applied more widely. That is, Channel State Information (CSI) is acquired in advance by uplink channel estimation for downlink precoding/beamforming transmission.
Closed loop MIMO transmission by use of uplink/downlink channel reciprocity in a 5G communication system is also confronted with many technical challenges. Two main challenges are the channel aging problem and the problem of limits to the number of uplink reference signals, for example, channel SRSs. Since a channel aging effect of a 4G system is not so serious, latencies among an SRS, codebook feedback and signal transmission of an LTE system may not exceed a coherence time of a channel even though they are relatively long. Therefore, the LTE system is not specially optimized to shorten a latency from channel measurement to data transmission. For either a periodic SRS or an aperiodic SRS, the SRS is configured on a last symbol of each uplink subframe only.
However, in the 5G system, a channel aging effect may be greatly intensified and a coherence time may also be greatly shortened.